V. A. Gorshkov

High-temperature synthesis of cast Cr2AlC at an inert gas overpressure

This paper reports the high-temperature synthesis of cast Cr2AlC from mixtures of chromium(III) oxide, chromium(VI) oxide, aluminum, and carbon at an increased inert gas (Ar or N) pressure in a 3-L self-propagating high-temperature synthesis reactor at an initial argon pressure pi = 5 MPa. The reaction products were characterized by X-ray diffraction and X-ray microanalysis. The results indicate that the ingots obtained under optimized conditions consisted of Cr2AlC and Cr3C2 with a ratio of about 85 : 15.

Cast silicides of molybdenum, tungsten, and niobium by combustion synthesis

Cast silicides of molybdenum, tungsten, and niobium were prepared by combustion synthesis under nitrogen pressure from powder mixtures of respective metal oxides with Al and Si. Upon variation in green composition, cast MoSi2, WSi2, NbSi2, and their solid solutions with variable phase composition have been synthesized. NbSi2 was obtained by combustion in the presence of added heat-generating CaO2-Al powder mixture.

SHS Casting of (Mo,W)Si2, (Mo,Nb)Si2, and (Mo,Ti)Si2 silicides: effect of activating 3CaO2 + 2Al additives

Abstract(Mo,W)Si2, (Mo,Nb)Si2, and (Mo, Ti)Si2 silicides and their solid solutions were prepared by the technique of SHS casting under 5 MPa of Ar pressure from thermit-like green mixtures containing the oxides of Mo, Nb, and Ti as well as activating 3CaO2 + 2Al additives. The synthesized composites were characterized by XRD and EDS.

Oxynitrides by aluminothermic SHS in nitrogen gas: Influence of nitrogen pressure

Cast aluminum oxynitrides were prepared by aluminothermic SHS under pressure of nitrogen gas and characterized by time-resolved XRD and chemical analysis. Burning velocity U, material loss caused by sputtering η1, and yield of target material into ingot η2 were measured as a function of nitrogen pressure P(N2). The nitrogen content of synthesized oxynitrides was made as high as 7 wt %. The results may turn helpful in designing high-strength transparent ceramics.

Aluminothermic SHS in CaCrO4–Al–C Mixtures under Nitrogen Pressure

η1 Refractory Cr compounds – such as Cr23C6, Cr7C3, Cr3C2, and Cr2AlC – known for their high hardness, strength, and also wear, heat, and corrosion resistance [1, 2] are widely used in deposition of protective coatings [3–5]. On an industrial scale, chromium compounds are fabricated by the methods of powder metallurgy [6–8]. Such materials can also be obtained from CrO3–Al–C mixtures by metallothermic SHS under gas pressure [9–11]; but a main drawback of this technique was the water absorbency and thermal instability of CrO3. In this work, we made an attempt to replace CrO3 by calcium chromate CaCrO4 that exhibits a low water absorbency and high heat resistance. The commercial powders of CaCrO4 (reagent grade), Al (ASD-1 brand), and graphite (GMZ brand, d = 40–160 μm) we used to prepare (CaCrO4 + Al) + nC mixtures, where n stands for a wt % of graphite in CaCrO4 + 2Al mixtures. Green mixtures were prepared by intermixing either in a planetary or in a ball mill. 20 g of the above mixtures were placed in a silica cartridge (15–25 mm in diameter, 50–60 mm long) and ignited from top in a closed 3-L reactor under 5 MPa of nitrogen gas. The values of burning velocity U, yield η1 of metallic phase, and extent of product splashing η2 were measured as described elsewhere [9, 10]. Combustion products were characterized by XRD (DRON-3 diffractometer) and high-resolution SEM/EDS (ULTRA plus Zeiss microscope with INCA 350 accessory, Oxford Instruments). Preliminary thermodynamic calculation (ISMAN TERMO software) has shown that an increase in n (up to 3.7%) can be expected to decrease Тad, the yield of the oxide phase, and to improve the yield of the metallic phase. Green mixtures with n up to 3.7 wt % burned in a mode of frontal combustion and the cast combustion products could be easily separated into a metallic (tar-

Chemical and Phase Transformations in the Combustion of a CrO3/AlN Mixture

The possibility of combustion of a mixture of chromium peroxide with aluminum nitride is shown experimentally. The effect of initial pressure of nitrogen and ratio of reagents on an average linear burning rate, as well as on a relative mass loss in the combustion of a CrO3/AlN mixture is studied. The concentration limits of the burning rate of the test mixture are determined. The microstructure and phase and chemical compositions of the combustion products of the chromium–nitride-aluminum mixture are described.

AlON powders by aluminothermic SHS under pressure: Synthesis and characterization

Heat-resistant and chemically inert xAlN–yAl2O3 solid solutions, also known as AlONs, find their application in production of optical ceramics [1, 2]. γ-AlON powders as raw materials for fabrication of transparent ceramics are currently obtained by hot pressing and sintering [3], carbothermal reduction [4], solid-phase reaction [5], and spark plasma sintering [6]; the processes of carbothermal reduction and solid-state reaction being most widespread [7]. AlON powders were also prepared by metallothermic SHS from Al–Al2O3 mixtures [8–10] and also from a number of other thermit-type mixtures [11–14] and showed good mechanical and refractory behavior [15].

Cast ceramics by metallothermic SHS under elevated argon pressure

Cast ceramic composites were prepared by metallothermic SHS under elevated Ar pressure (5 MPa). Variation in green composition was used to affect the phase composition of combustion products. Under some optimized conditions, resultant ceramics can be obtained in the form of Al2O3–Cr2O3 solid solutions, Al2O3–Cr2O3 ∙ хZrO2 composites or single-phase Al2MgO4. Such materials seem promising for use in jewelry, process engineering (casting molds, cutting tools), and aerospace industry.

Synthesis of aluminum oxynitride from MoO 3 /Al/AlN mixtures by SHS metallurgy

This work investigates the regularities of autowave chemical transformation of MoO3/Al/AlN mixture, gravity separation of its combustion products, and formation of composition and structure of end product (aluminum oxynitride). It is demonstrated that the mixtures of MoO3, Al and AlN are capable of burning in wide ranges of reagent ratios. In the general case, this process is composed of three consecutive stages: stage 1—combustion and chemical transformation of MoO3, Al, and AlN mixture into two-phase melt of end products (autowave synthesis); stage 2—gravity separation of combustion products (metallic and oxide phases), where two layers are generated: upper layer of “light” oxynitride (Al2O3–AlN) and bottom layer of “heavy” intermetallic compound (Mo–Al); stage 3—cooling and generation of phase composition and crystalline structure of metallic and oxide layers. It is established that variation of the aluminum content in the initial mixture at constant content of nitride additive significantly influences the combustion and autowave chemical transformation of the mixture and the phase composition and structure of the end product (aluminum oxynitride). Optimum compositions are determined for obtaining aluminum oxynitrides with high nitrogen content (up to 6–9 wt %) and impurity content (Al7O3N5 and AlN) below 3 wt %. The obtained cast aluminum oxynitride and method of its production are promising for production of high-strength transparent ceramics.

High-temperature synthesis of a cast material with a maximum content of the MAX phase Cr2AlC

Experimental data are presented on the high-temperature synthesis of cast materials in the Cr–Al–C system with a maximum content of the MAX phase Cr2AlC. Experiments were carried out in multipurpose self-propagating high-temperature synthesis (SHS) reactors at an argon pressure p = 5 MPa. The starting mixtures consisted of chromium(VI) and chromium(III) oxides, aluminum, and carbon. It has been shown that varying synthesis parameters may have a significant effect on the phase composition and microstructure of the final products. We have optimized synthesis conditions for the preparation of materials with a maximum content of the MAX phase Cr2AlC and assessed the effect of acid media on the phase composition of the synthesis products. A material based on the MAX phase Cr2AlC has been shown to have high resistance to aqueous 30% HCl, 10% HF, and 45% HF solutions. The materials prepared by the SHS metallurgy process have been characterized by X-ray diffraction, X-ray microanalysis, and microstructural analysis.